Method and system for communicating unicast/multicast messages using automatic dependent surveillance - broadcast (ads-b) protocol

ABSTRACT

Methods are described for generating encrypted messages and unicast/multicast transmitting the encrypted messages to one or more aircraft using Automatic Dependent Surveillance-Broadcast (ADS-B) transmission. Corresponding system for communicating messages to an aircraft using ADS-B link is also provided.

FIELD OF THE INVENTION

The present application relates to communicating unicast/multicastmessages using an Automatic Dependent Surveillance-Broadcast (ADS-B)protocol, in particular for application to the art of enabling asafeguard communication in the aeronautics industry, and, in particular,to a method and system for communicating flight control commands to anaircraft, for example to unmanned aircraft (UA) and Ground Stations(GS).

BACKGROUND OF THE INVENTION

Currently, the UAs are operated according to FIG. 1. The UA 100 iscontrolled by an on-board autopilot 104 which receives the flightcontrol commands from a ground radio 108 through a bidirectional radiolink 112. The Ground Control Station 102 comprises a processing computerPC 106 for processing the data obtained from the ground radio 108, andto generate flight control commands. The flight control commandsgenerated by the processing computer 106 are provided to the radio 108via communication link 110. The flight control commands received by theradio 108 are transmitted via radio frequency telemetry link 112 to theUA 100, to be referred to as radio link for simplicity. This radio linkis short range which does not allow to control the UA from remotelocations, when, for example, a third party needs to communicate to theUA.

Another drawback is the risk of a “fly-away” UA. When a radio controllink loss occurs, an autonomous UA can fly on its own, posing dangers tothe general public on the ground, as well as to other aircraft (mannedand unmanned) in its vicinity and over great distances. In the case whenthe control link is lost, the Pilot-In-Command (PIC) should still beable to mitigate the risks of collision: certain collision avoidancemaneuvers must be communicated to the UA and then further executed bythe onboard autopilot.

One prior art solution is shown in FIG. 2, where a secondary radio link114 is integrated in the system to connect the UA 100 to the GroundControl Station 102. The secondary radio link 114 is a redundant linkwhich is used in the case when a primary radio link 112 is lost. Twosolutions are viable: to establish the secondary radio link 114 on thesame frequency as the primary radio link 112, which provides theredundancy of the hardware; or use a secondary radio link 114 of a lowerfrequency, which could decrease the performance of the primary link 112.Also, considering the lack of the space on board of a small UA, tworadio frequency (RF) links positioned close to each other could createinterference. Another disadvantage of such approach is that thesecondary radio link 114 may be lost by being blocked by tall objects inthe operation area of the UA. Another potential disadvantage is that thesame operational conditions which lead to the loss of connectivity ofthe primary radio link 112 may also cause a failure in connectivity forthe secondary radio link 114.

The loss of radio link needs to be addressed as a link-loss procedure oremergency situation. It is important that an aircraft always operates ina predictable manner. Moreover, it is important to know a position ofthe aircraft at the time of link-loss, and execute an emergency maneuverwhich does not pose any danger to humans, private property, otheraircraft, etc. It is also important that the third party can send anycommands to the UA from remote locations where primary radio link is notwithin the communication range. Link loss in current description is onlyone example of an emergency situation, and the principles of theinventions can be applied in other situations.

Currently available are established aeronautical technologies: AutomaticDependent Surveillance-Broadcast (ADS-B) transmission protocol andUniversal Access Transceivers (UAT) radio hardware. ADS-B is used byaircraft and certain equipped ground stations to share flightinformation, and UAT is a multi-purpose aeronautical data link intendedto support ADS-B and other flight and traffic information services.

The ADS-B is a surveillance technology in which an aircraft determinesits position via satellite navigation and periodically broadcasts theposition of the aircraft, enabling the position to be tracked. Theinformation can be received by Air Traffic Control Ground Stations as areplacement for secondary radar as no interrogation signal is neededfrom the ground. It can also be received by other aircrafts to providesituational awareness and allow self-separation. ADS-B is “automatic” inthat it requires no pilot or external input. It is “dependent” in thatit depends on data from the navigation system of the aircraft.

ADS-B has two different services, “ADS-B Out” and “ADS-B In”, andenhances safety by making an aircraft visible, in real-time, to AirTraffic Control (ATC) and to other appropriately equipped ADS-B aircraftwith position and velocity data transmitted periodically. ADS-B data maybe recorded and downloaded for post-flight analysis. ADS-B also providesdata infrastructure for inexpensive flight tracking, planning, anddispatch.

“ADS-B Out” periodically broadcasts information about each aircraft,such as identification, current position, altitude, and velocity,through an onboard transmitter. ADS-B Out provides air trafficcontrollers with real-time position information that is, in most cases,more accurate than the information available with current radar-basedsystems. With more accurate information, ATC can position an aircraftwith improved precision and timing.

“ADS-B In” is the reception by aircraft of Flight InformationServices-Broadcast (FIS-B) and Traffic Information Service-Broadcast(TIS-B) data and other ADS-B data such as direct communication fromnearby aircraft. Ground station broadcast data is typically only madeavailable in the presence of an ADS-B Out broadcasting aircraft,limiting the usefulness of purely ADS-B In devices.

The ADS-B technology relies on two avionics components, high-integrityGPS navigation source and a datalink (ADS-B unit). There are severaltypes of certified ADS-B data links, but the most common ones operate at1090 MHz, or at 978 MHz (UAT).

However, neither the existing radio link technology nor ADS-B technologycould address a problem of more reliable communication with an aircraft,in particular in emergency situations, while remaining compliant withexisting aviation standards and avoiding the need of replacing existinghardware equipment at both ground stations and aircrafts.

Therefore, there is a need in the industry for developing an improvedsystem and method for sending control commands to an aircraft fromseveral locations, including remote locations of third-parties, tomitigate a risk in emergency situations such that collision with mannedaircraft, fly-away, radio link loss etc. while addressing air privacyconcerns.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide an improvedmethod and system for communicating flight control commands to/fromunmanned aircraft or aerial vehicles (UAVs), thus improving aircraftsafety.

According to one aspect of the invention, there is provided atransmission method from a first entity to a second entity via anAutomatic Dependent Surveillance-Broadcast (ADS-B) transmissionprotocol, comprising: generating a message for transmission from thefirst entity to the second entity; encrypting said message with a uniqueset of keys associated with the first entity and the second entity togenerate an encrypted message; generating a header for identifying amessage type; generating a sub-header having at least one of a bitmap, atarget indicator, a public key and a destination address; and forming anADS-B Universal Access Transceiver (UAT) frame comprising a headerportion and a payload portion wherein said header is mapped into theheader portion and said encrypted message and said sub-header are mappedinto the payload portion; and transmitting said ADS-B UAT frame to saidsecond entity thereby converting the ADS-Broadcast transmission protocolinto a unicast/multicast transmission.

In the method described above, the encrypted message and sub-header arecarried on reserved bits of an ADS-B UAT frame having a payload typevalue comprised between 7 and 10. Alternatively the encrypted messageand sub-header may be carried on a reserved message of an ADS-B UATframe having a payload type value comprised between 11 and 29.

In the method, the first entity is one of a ground station and anAuthorities control station and said second entity is one of anaircraft. Alternatively said first entity is one of a ground station andan Authorities control station and said second entity is a group ofaircraft. Yet in another alternative the first entity is an aircraft andsaid second entity is one of a ground station and an Authorities controlstation.

According to another embodiment, the encrypting step first encrypts themessage with the first entity private key to generate a first encryptedmessage and then encrypts the first encrypted message with the secondentity public key to generate the encrypted message and wherein saidunique set of keys comprise the first entity private key and the secondentity public key. Alternatively, the encrypting step first encrypts themessage with the second entity public key to generate a first encryptedmessage and then encrypts the first encrypted message with the firstentity private key to generate the encrypted message and wherein saidunique set of keys comprise the first entity private key and the secondentity public key.

Furthermore, the sub-header comprises the bitmap, the target indicatorand the public key of the first entity. According to a furtherembodiment, the message is one of a flight control command, an emergencycommand and an informational message.

Additionally, the message is selected from a menu of predeterminedmessages.

According to another aspect of the invention there is provided a systemfor communicating between a first entity to a second entity viaAutomatic Dependent Surveillance-Broadcast (ADS-B) transmissionprotocol, the system comprising: an encryption module for applying a2-layer encryption to a command and control (C&C) message to generate anencrypted C&C message wherein said 2-layer encryption is based on aunique set of keys associated to the first entity and to the secondentity; a header generation module for generating a header comprising anaddress qualifier; an ADS-B Universal Access transceiver (UAT) frameformatter for generating a sub header comprising at least one of abitmap, a target indicator, a public key and a destination address; saidADS-B UAT frame formatter further forming an ADS-B UAT frame comprisinga payload portion and a header portion wherein said header portionincludes said generated header and said payload portion includes saidsub-header and said encrypted C&C message and wherein said addressqualifier indicates that said ADS-B UAT frame carries a C&C message; andtransceiver for transmitting said ADS-B UAT frame to the second entity.

In the system described above, said first entity is one of a groundstation and an Authorities control station and said second entity is oneof an aircraft.

The unique set of keys of the system may comprise a private key of thefirst entity and a public key of the second entity.

Furthermore, the private key is applied on the C&C message to generate afirst encrypted message and the public key is applied on said firstencrypted message to generate the encrypted C&C message.

In one aspect, the encrypted C&C message and sub-header are carried onreserved bits of an ADS-B UAT frame having a payload type valuecomprised between 7 and 10. Alternatively the encrypted C&C message andsub-header are carried on a reserved message of an ADS-B UAT framehaving a payload type value comprised between 11 and 29.

Additionally, the C&C message is one of a flight control command, anemergency command and an informational message.

The system comprises a control processing system at the second entityfor receiving said transmitted ADS-B UAT frame. Furthermore, the controlprocessing system comprises an ADS-B frame parser for parsing the ADS-BUAT frame received at said second entity. Additionally, the controlprocessing system further comprises a decryption module for decryptingthe encrypted C&C message to retrieve the C&C message and said controlprocessing system further forwards said C&C message to an autopilot ofthe second entity.

According to yet another aspect of the invention, there is provided atransmission method from a ground station to an aircraft via anAutomatic Dependent Surveillance-Broadcast (ADS-B) transmissionprotocol, comprising: generating a command and control (C&C) message fortransmission to the aircraft responsive to a detection of a radio linkloss situation between the ground station and the aircraft; encryptingsaid C&C message with a unique set of keys associated to the groundstation and to the aircraft to generate an encrypted C&C message;generating a header for identifying a message type, wherein an addressqualifier of said header indicates the message type as a C&C message;generating a sub-header having at least a bitmap, a target indicator,and a public key; and forming an ADS-B Universal Access Transceiver(UAT) frame comprising a header portion and a payload portion whereinsaid header is mapped into the header portion and said encrypted C&Cmessage and said sub-header are mapped into the payload portion; andtransmitting said ADS-B UAT frame to said aircraft thereby convertingthe ADS-Broadcast transmission protocol into a unicast/multicasttransmission.

According to one more aspect of the invention, there is provided asystem for communication using an ADS-B transmission protocol, thesystem comprising a processor, and a memory device having computerexecutable instructions stored thereon, causing the processor to:generate a command and control (C&C) message for transmission to areceiving entity responsive to a detection of a radio link losssituation between said receiving entity and a transmitting entity;encrypt said C&C message with a unique set of keys associated to thereceiving entity and the transmitting entity; generate a header foridentifying a message type, wherein an address qualifier of said headerindicates the message type as a C&C message; generate a sub-headerhaving at least one of a bitmap, a target indicator, a public key and adestination address; and form an ADS-B Universal Access Transceiver(UAT) frame comprising a header portion and a payload portion whereinsaid header is mapped into the header portion and said encrypted C&Cmessage and said sub-header are mapped into the payload portion; andtransmit said ADS-B UAT frame to said receiving entity therebyconverting the ADS-Broadcast transmission protocol into aunicast/multicast transmission.

Thus, an improved method and system for communicating commands andcontrol messages from/to an aircraft have been provided.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the embodiments and/or relatedimplementations described herein and to show more clearly how they maybe carried into effect, reference will now be made, by way of exampleonly, to the accompanying drawings which show at least one exemplaryembodiment and/or related implementation in which:

FIG. 1 illustrates a prior art communication system for communicatingwith an aircraft;

FIG. 2 illustrates another prior art communication system forcommunicating with an aircraft;

FIG. 3 illustrates a communication system for communicating with anaircraft according to an embodiment of the invention;

FIG. 4 illustrates a communication system for communicating with anaircraft according to another embodiment of the invention;

FIG. 5A illustrates a graphical user interface for selecting command fortransmission according to one embodiment of the invention;

FIG. 5B illustrates a method for receiving communication messages at anaircraft from a Ground Control Station of an embodiment of the inventionfollowed by further actions;

FIG. 6 illustrates an architecture of the GS control and processingsystem;

FIG. 7A, illustrates a table showing the frame format, different fieldsand different message types of and ADS-B UAT frame;

FIG. 7B illustrates a table showing the different address qualifiers andcorresponding types of an ADS-B UAT frame;

FIG. 7C illustrates the composition of an ADS-B UAT header;

FIG. 8A illustrates the frame structures according to a first embodimentof the invention;

FIG. 8B illustrates the frame structures according to a secondembodiment of the invention;

FIG. 9 illustrates a signal flow depicting the processing of a commandand control message at transmission and reception;

FIG. 10A illustrate an encryption operation of the command and controlmessage; and

FIG. 10B illustrates a decryption operation of the command and controlmessage.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 3 illustrates a communication system 10 for communicating commandand control messages to/from an aircraft, according to the embodiment ofthe invention. By way of an example, the aircraft is an unmannedaircraft (UA) 100 which is controlled by an autopilot 104. In thepresent application, the unmanned aircraft 100 is any aircraft operatedby an autopilot, which in turn is controlled from a ground stationremotely by the Pilot-in-Command or PIC.

The communication system 10 comprises a Ground Control Station 102,which is in communication with the UA 100. The Ground Control Stationcomprises the radio link 112 and a ground station (GS) transceiver 120,controlled by a Ground Station Control and Processing System 132Adetailed description of the GS Control and processing System 132 will beprovided later in the description of FIG. 6.

A person of ordinary skill in the art would recognize that in thisembodiment, the control and processing system 132 can be implemented asa stand-alone system without the need to modify the software running onthe existing processing computer 106 of the Ground Control Station 102of FIGS. 1 and 2. In a further embodiment, processing computer 106 cancomprise a memory device storing additional software for performingfunctions of the control and processing system 106 a as shown in FIG. 4.

The unmanned aircraft 100 comprises the autopilot 104 and an UAtransceiver 122, both controlled by an Onboard control and processingsystem 124 at the UA 100 as shown in FIG. 3. The Onboard control andprocessing system 124 is provided for control of emergency situation andfor communicating with the ground station control and processing system132 or other entities according to the principles of the invention.Conveniently, the ground transceiver 120 and the UA transceiver 122 maybe Universal Access Transceivers (UAT) for example.

The embodiment of the present invention incorporates the AutomaticDependent Surveillance-Broadcast (ADS-B) transmission protocol toprovide an additional ADS-B compliant communication link 121 between theGS transceiver 120 and the UA transceiver 122, thereby providing anadditional communication channel between the Ground Control Station 102and the UA 100. The present invention transforms ADS-B that was designedfor broadcasting flight data into a point-to-point orpoint-to-multipoint communication system for any type of message and inparticular for aircraft command and control messages. A method andsystem are thus provided to achieve this unicast or multicastcommunication via ADS-B protocol, while remaining compatible withindustry standardized UAT transceiver hardware. This is achieved byencrypting part of the ADS-B message and transmitting it from Groundstation 102 to the UA 100, or vice versa and by providing correspondingcontrol and processing systems at both ground station 102 and UA 100while keeping unchanged the frame structure of the ADS-B UAT frame.Multicast, in the current applications, means communicating the sameencrypted message to a selected number of entities which share sameencryption keys.

The ADS-B communication link 121 is in addition to the primary radiolink 112 between the radio 108 and the UA 100. The ADS-B communicationlink 121 is used to transmit command and control messages from/to theground transceiver 120 to/from the UA transceiver 122.

Since the ADS-B transmission protocol uses a low frequency andhigh-power communication link, it is more reliable compared to atraditional RF telemetry link 112. The ADS-B protocol technology usesGPS to determine the position of the aircraft 100.

When the radio link 112 is lost, there is no communication between theradio 108 of Ground Control Station 102 and the autopilot 104 of theaircraft 100. However, the ADS-B communication link 121 between theground transceiver 120 and the UA transceiver 122 at the aircraft 100 isstill present.

An action of the PIC invokes sending an encoded command and controlmessage 115 from the GS control & processing system 132 to the GStransceiver 120. Upon receiving the encoded command and control message115 formatted according to the principles of the invention, the groundtransceiver 120 sends the message thus formatted to the UA transceiver122 via ADS-B link 121 that only a specific UA 100 can decrypt.

In one embodiment of the present invention, the PIC selects a commandand control message, out of a plurality of messages. Once the commandand control message is selected, the command is encoded and formattedaccording to the principles of the invention before being provided tothe GS transceiver 120 for transmission to the UA 100. This operationwill be further described below.

Once the encoded command and control message 115 is transmitted by theGS transceiver 120 as the transmitted encoded command and controlmessage over the ADS-B communication link 121, it is received by the UAtransceiver 122 at the aircraft 100 and provided, via communication link123, to the onboard control & processing system 124. The contents of theencoded command and control message 115 transmitted over the ADS-Bcommunication link 121 are processed by the onboard control & processingsystem 124. After receiving and decoding the encoded command and controlmessage 115 to retrieve the command and control message, the UAtransceiver 122 will send an “acknowledgement” message to the GroundStation 102, followed by a control action to the autopilot 104 toexecute a required emergency maneuver.

In an embodiment of the present invention, the onboard control &processing system 124 decodes the transmitted encoded command andcontrol message sent over the ADS-B communication link 121 with a pairof unique encoding or encryption/decryption keys. The acknowledgemessage is then sent to the ground station, and the emergency flightcommand 125 is provided to the autopilot 104 to execute the requiredemergency maneuver.

FIG. 4 illustrates yet another embodiment of the present inventionincluding an Authority Control Station control and processing system 134for handling operation when an emergency situation arises. In FIG. 4,similar elements have been designated with the same reference numeralsas shown in FIG. 3, with an additional authorities control station 130.

The PIC action is shown in FIG. 4 relying on the link 116 between the GScontrol & processing system 132 and the GS transceiver 120, to send theencoded command and control message 115 generated in response to the PICpressing a button on a software interface.

The embodiment described in FIG. 4 further allows for a third-partyemergency action. Currently, a UTM (Unmanned Traffic Management) systemis being implemented by the Federal Aviation Administration (FAA),National Aeronautics and Space Administration (NASA), and the industry,for controlling operations of unmanned aircraft. Authorities, emergencyservices, and law enforcement have access to the information about amission of an unmanned aircraft, to have a chance to warn other airspaceusers, and perform various actions as required. This is shown in FIG. 4as the authorities control station 130 being in communication with theaircraft 100 via yet another ADS-B communication link 137 betweenanother ground transceiver represented as Authorities Control Station(ACS) transceiver 136 at the authorities ground station 130 and the UAtransceiver 122, which allows the authorities to perform the emergencyaction in case the PIC is not responding to the emergency situation.

In one embodiment, the authorities may decide to generate, using thecontrol and processing system 134 a, an authority action encoded flightcontrol message 133 to be transmitted over a link 135 from theauthorities control and processing system 134 to the ACS transceiver136, which is further transmitted as the transmitted encoded flightcontrol message through the ADS-B communication link 137. Thetransmitted encoded flight control message is received by the UAtransceiver 122 at the UA 100 and provided via communication link 123 tothe onboard control and processing system 124. The contents of thereceived encoded flight control message are processed by the onboardcontrol and processing system 124, the acknowledgement message is sentback to the ground station, followed by a control action given to theautopilot 104 to execute a desired emergency maneuver.

In one embodiment, the encoded command and control message 133 has ahigher priority than other communications between the ground controlstation 102 and the aircraft 100. In an embodiment of the presentinvention, the onboard control and processing system 124 decodes thereceived encoded command and control message with a pair of unique keysto recover the command and control message represented on UA side asemergency flight command 125 which corresponds to the control actiongiven by the authorities ground control station 130. The emergencyflight command 125 is provided to the autopilot 104 to execute thedesired emergency maneuver.

Although the present embodiments have described the communicationbetween the ground control station 102 and the authorities controlstation 130 with a single unmanned aircraft 100, it should be understoodthat a single ground station 102 may control one or more unmannedaircrafts, in a similar fashion by communicating with respectiveaircraft using encrypted messages that are generated by using encryptionkeys that are specific to each of the aircraft and ground station forunicast transmission or by using a group key specific to a groupconsisting of one or more unmanned aircraft and the ground station formulticast transmission. Similarly, a single authorities control station130 may control one or more unmanned aircrafts, in a similar fashion bycommunicating with respective aircraft using encrypted messages that aregenerated by using encryption keys that are specific to each of theaircraft and ground station codes. Similarly, PIC and authorities groundstations may have similar set of commands which allows to communicatebetween their ground stations, acknowledge receiving and decoding themessage, etc.

An example of a graphical user interface 500 for the control andprocessing system 132 is given in FIG. 5A for entering commands to sendto UA 100 or to a group of aircraft. In one embodiment of the presentinvention, communication between the control and processing system 132of ground control station 102 and the GS transceiver 120 (FIG. 4) isdetermined by the “Communication settings” window 502 in FIG. 5A. BothGS transceiver 120 and UA transceiver 122 are assigned respectivespecific International Civil Aviation Organization (ICAO) numbers, whichhowever are not limited, and any other unique identifiers could be alsoused. The system needs to set ground station and/or aircraft ICAOnumbers (or any other unique identifiers) 510 and 508, so that it candistinguish to which transmitters command and control messages are to besent. The most suitable emergency command 506 may then be selected orchosen by the PIC depending on the situation. An execution of theemergency command 506, pressing the button “Emergency” 514, results in acommand and control message such as an emergency flight command beingtransmitted to the UA 100 according to a method of the invention. In oneembodiment, after decoding the received encoded command and controlmessage with the pair of unique keys at the UA 100, communication fromthe onboard control and processing system 124 to the autopilot 104 inFIG. 4 is initiated to forward the retrieved emergency flight command tothe autopilot 104.

In one embodiment, the emergency command 506 field can comprise sixcharacters that can include both letters and numbers representing aselected command and control message that PIC intends to send to thetarget entity. In a further embodiment, different strings with differentnumber of characters and composition may be implemented to represent thecommand and control message to be entered on emergency command 506.Thus, several flight control commands may be chosen as emergencymaneuvers by the PIC, e.g. loitering, hovering, emergency land, variouscollision avoidance maneuvers—turns, altitude change etc. An emergencymaneuver may be selected as shown in the emergency command 506 in FIG.5A, assuming commands “KILL01”, “LAND02” etc., in the window 206indicate a certain maneuver.

Although in FIG. 5A of this description a software interface is shown,other interfaces to input the PIC command and control messages can beused within the scope of the invention.

FIG. 5B illustrates a generic operation at the aircraft 100, including amethod 520 for receiving the encoded flight control message at UAtransceiver 122 of the UA100 (box 530), decoding the encoded flightcontrol message with the pair of unique encryption keys at the onboardcontrol and processing system 124 to retrieve the emergency flightcommand (box 540), sending an acknowledgement message back to the groundstation 102, and providing the emergency flight command to the autopilot104 instructing the autopilot 104 to perform an emergency maneuver (box550). In one embodiment, once the radio link to the ground station isre-established, the on-board emergency processing system enable the PICto take back the control by overriding the emergency flight controlcommands received from the authorities ground stations (Box 560).Alternatively, the authorities ground station 130 has priority over thePIC and will need to relinquish control for the PIC to take back controlof the UA 100.

FIG. 6 shows an expanded diagram for the system 10 of FIG. 3 with thecontrol and processing system 132 shown in greater detail, and wherecertain components not necessary for the description of this embodimenthave been omitted so not to obscure the description. Operations foremergency control in situation of link loss situation will be describedfrom the perspective of the ground station 102 however the samecomponents and functionalities are present in the control and processingsystem 134 of the Authorities Control station 130.

In one embodiment, the UA 100 includes an Onboard control and processingsystem 124 to be able to communicate with the ground station 102 orother aircraft implementing this invention. The components andfunctionalities of the UA Onboard control and processing system 124 arethe same as for the GS control and processing system 132.

Additionally, the communication method and system disclosed herein applyto non-emergency situations and can be applicable to any bidirectionalcommunication between the ground station 102 or Authorities Controlstation 130 and one or more aircraft.

As disclosed previously, the ground station 102 communicates with theunmanned aircraft 100 primarily through the radio link 112 and, uponloss of that radio link 112, switches to the ADS-B UAT link 121 totransmit command and control (C&C) messages. A C&C message is definedand used in the present application in a broad sense and can be anymessage transmitted through the communication method and systemdisclosed in this application, and can be flight control commands,emergency commands and other PIC control and informational messages.

As shown in FIG. 6, the e ground station (GS) 102 includes the groundstation transceiver 120. In this embodiment, the GS transceiver 120 is aUniversal Access Transceiver (UAT) for transmitting ADS-B framesgenerated from the GS control and processing system 132. The GS controland processing system 132 comprises a link monitoring system 620 formonitoring the radio link 112 (shown in FIG. 3), and upon detection of alink loss alerts a PIC for taking control and starting the emergencyoperation mode when needed.

The GS control & processing system 132 at the ground control station 102further comprises a memory 626 storing code for performing operation ofthe control and processing system 132 to be executed by the processor625. The control and processing system 132 initiates a process fortransmitting an encrypted message or command from the ground controlstation 102 to the UA 100.

The processor 625 of the GS control and processing system 132 presentsan interface to the PIC to interact with the GS control and processingsystem 132. Such interface can be the graphical user interface 200disclosed in FIG. 5A. The PIC can use the graphical user interface 200to enter its command and the processor 625 initiates the generation ofthe ADS-B UAT C&C message corresponding to that command according to theprinciples of this invention. In this embodiment, the processor 625after receiving the command entered through the GUI 200, interacts withthe encryption/decryption module 622 to encrypt the C&C message.

The encryption/decryption module 622, on the transmit side applies a2-layer encryption method to generate the encrypted C&C message. On thereception side the encryption/decryption module 622 deciphers theencrypted C&C message to recover the corresponding message. Theoperation of the encryption/decryption module 622 will be describedbelow with regards to FIGS. 10A and 10B.

The control and processing system 132 relies on a Header Generator 624to generate a header and on the ADS-B frame formatter/parser 621 to forman ADS-B UAT frame carrying the encrypted C&C message according to theprinciples of the invention. The ADS-B UAT frame thus formed comprises aheader portion and a payload portion which includes a sub-header and theencrypted C&C message.

The ADS-B UAT frame of the present invention is formatted using the sameframe structure as a standard ADS-B UAT frame, making this inventionbackward compatible with the standard ADS-B UAT frame. However, newmessage types and methods to package the messages into an ADS-B UATframe are introduced in the current disclosure.

The description of the Header generator 624 and the ADS-B frameformatter/parser 621 will be based on FIGS. 7A, 7B and 7C and on FIGS.8A and 8B, respectively.

As shown in FIG. 7A, ADS-B-UAT define potentially 32 messages (0-31)identified by the payload type code, however in reality only 11 (0-10)messages are currently defined.

The ADS-B UAT frame structure has a 4-byte header, and a message payloadhaving a state vector (SV) which is the aircraft position informationand additional information field such as reserved bits and mode status(MS).

Payload types 7-10 define messages with header field and SV field alongwith reserved bits (element 720). Payload types 11-29 are reserved forfuture use and are comprised of the header and of the entire payloadfields (element 730) reserved for future needs and are referred hereinas reserved messages. Messages 30, 31 are meant for developmental use(for example, to test a new message structure).

The present invention uses these undefined or reserved bits (element710) of payload type 7 to 10 or reserved messages (element 720) ofpayload types 11-29 to introduce new message types and thus newfunctionalities not originally contemplated by ADS-B UAT while at thesame time preserving the header structure and payload structure of theADS-B UAT to maintain compatibility with devices not implementing thepresent invention.

The header of the ADS-B UAT 740, as shown in FIG. 7C, contains thefollowing information: payload type code, address qualifier, andaddress. The first 5 bits of Byte 1 of the header are used to encode thepayload type while the last 3 bits of Byte 1 are used to encode anaddress qualifier and the remaining 3 Bytes (or 24 bits) are used toencode the address.

Payload type code is the number from 0 to 31 shown in FIG. 7A. Addressis a 24-bit address used in conjunction with address qualifier.Typically for an aircraft, this address is a 24-bit ICAO number assignedto each aircraft upon its registration. This number is programmed intothe UAT unit, and it is illegal to modify it. Address qualifier, insimple terms is what type of target the message is coming from.

FIG. 7B shows all possible types of address qualifiers. There arereserved address qualifiers (RAQ) “110” and “111” represented in FIG. 7Bas elements 730 a and 730 b respectively which are the binary addressesfor reserved types 6 and 7, respectively of FIG. 7B. These RAQ 730 a and730 b have been used in the embodiments of the invention to indicatethat the message is coming from an entity (ground station or aircraft)implementing the present invention and in communication with theaircraft in question.

As an example, the RAQ 730 a for the binary address qualifier “110” canbe used to indicate that the message is transmitted by a ground stationentity implementing this invention while the 111 RAQ 730 b for thebinary address qualifier “111” is used to indicate that the message iscoming from an aircraft entity implementing this invention, or viceversa. The use of these two reserved binary address qualifiers “110” and“111” indicates to its recipient a new message type corresponding to theC&C message and therefore an ADS-B UAT frame with such address qualifiercarries a C&C message. The use of the two reserved binary addressqualifiers “110” and “111” allows the system to define a new messagetype within the defined payload types of the existing ADS-B UATstandard.

In this embodiment, the reserved address qualifier is also used toindicate that the corresponding message should be interpreted accordingto the principles of the current invention, and therefore a standardADS-B UAT recipient would not be able to interpret the message asreserved address qualifiers “110” and “111” are currently not defined inthe ADS-B UAT standard.

In this embodiment, the Header Generator 624 creates the header 820 ofthe ADS-B UAT frame according to the principles described above andapplies the RAQ “110”, as an example, in the last 3 bits of the firstByte of the header 820 as shown in FIG. 7C.

The Header Generator 624 on the receiving side reads the header 820 anddetermines its content to identify the message type being received.

The ADS-B UAT frame formatter/parser 621, in one embodiment, uses thereserved Bytes 18-34 of an ADS-B message identified by one of thepayload types 7-10 (element 710 of FIG. 7A) to transmit the encryptedC&C message.

The ADS-B UAT frame structure according to this embodiment is shown inFIG. 8A. The payload 18-34 identified as element 800 is subdivided into2 sections a sub-header section 840 and an encrypted message section830. The sub-header section 840 is comprised of 4 fields.

The first field 840 a is a 4-bit Field of Byte 18 which contains abitmap for indicating if the following 4 fields have content associatedwith them. An example bitmap of “1100” means that only the 2 fieldsfollowing the bitmap field have field value in the current message,namely the target indicator and the public key of the transmittingentity. This bitmap “1100” can be transmitted periodically or frequentlyby a transmitting entity to advertise its public key so that aircraftsor ground stations around can see the transmitting entity and can thensend encrypted messages to the transmitting entity, when required.

The second field 840 b of the sub-header 840 is a 1-bit field of aTarget Indicator for indicating whether the message being transmitted isdestined to a group or to an individual entity such as an aircraft or aground station.

The third field 840 c is the public key of the transmitting entity whichis in this embodiment of FIG. 6 the ground station. The size of thisthird field 840 c is set equal to a key size which can be 48 bits or anysuitable size. Setting a length of an encryption key is well known tothose skilled in the art.

The fourth field 840 d contains a Destination Address which can be aUnique Identifier to whom this message is addressed (Group name orIndividual name—e.g. ICAO). When the target indicator indicates a groupdestination, this field will contain a group name and when the targetindicator indicates an individual destination, this field will containan individual name. The presence of the destination address allowsaircrafts receiving this message to check first whether the message isaddressed to them before decrypting the encrypted message and thereforewill not waste computational resources trying to decrypt the message.

The sub-header 840 is disclosed here to comprise 4 fields, but thoseskilled in the can readily devise a different number of fields withoutdeparting from the invention. As an example, only three fields could beused by omitting the destination address field 840 d while stillmaintaining the functionality to convert the ADS-B broadcasttransmission protocol into a unicast or multicast transmission protocol,because of the encryption of the message with a unique set of keysassociated to the 2 entities in communication. The omission of thedestination address could be considered for the purpose of maximizingthe size of the encrypted message. Alternatively, an additional fifthfield may be inserted in the sub-header 840, to provide additionalinformation or additional functionalities as required.

In another embodiment, the public key field 840 c can be omitted as wellfrom the sub-header 840. In this embodiment, all entities incommunication have the public key of the other entities incommunication. This public key can be transmitted when no message isbeing sent corresponding to bitmap case of “1100” described above toallow all parties to learn and store the public key of the other partiessharing their public key. The public key field 840 c can be omitted bytransmitting an ADS-B message with the bitmap equal to “1011” or “1001”when the destination address is omitted. Other combinations can bedevised without departing from the invention.

The field 830 is for the encrypted C&C message generated by theencryption/decryption module 622. The ADS-B frame formatter/parser 621can then form the ADS-B UAT frame by composing all these fieldsincluding the header portion generated by the header generator 621 intoa frame as shown in FIG. 8A.

In one embodiment a frame size limiter 621 a is present within the ADS-BUAT frame formatter/parser 621 to ascertain that an ADS-B UAT frame thusformed by the ADS-B UAT frame formatter/parser 621 has a length equal toor less than the length of a standard ADS-B UAT frame as specified instandard specifications, which is currently 34 Bytes. Limiting the framesize can be done by adjusting the size of the encryption key, byomitting certain fields within the sub-header 840 as described above, byusing different encryption techniques, by encrypting only part of themessage, by inserting an index of the message instead of the entiremessage, or various other means.

In an alternative embodiment, the ADS-B frame formatter/parser 621 usesone of the reserved messages defined by payload types 11-29 (element720). The frame format according to this embodiment is shown in FIG. 8Band presents more bytes for transmitting the messages as the payloadextends from byte 5 to byte 34 compared to the frame format of FIG. 8A.FIG. 8B and FIG. 8A have the same fields however the payload size ofFIG. 8B is longer therefore longer encrypted messages can betransmitted. Alternatively, FIG. 8B can be used to transmit additionalsets of messages or longer encryption keys can be used to provide a morerobust encryption and thus a more secure transmission.

Once the ADS-B UAT frame is formed, it is transmitted through the GStransceiver 120 and subsequently received by the Onboard UAT transceiverrepresented as UA transceiver 122 at the UA 100 which processes ADS-Bframe through the Onboard control and processing system 124. Therecovered C&C message is then forwarded to the autopilot 104 asdescribed above to apply the command sent by the PIC. As stated above,the Onboard control and processing system 124 has identicalfunctionalities as the GS control and processing system 13 a and is aswell provisioned to process messages transmitted by the GS control andprocessing system 132. In particular, it can read the header through itsHeader Generator and parse the frame. The ADS-B frame formatter/parserwithin the Onboard control and processing system 124, when receiving aframe, parses the frame to identify the 4 fields of the sub-header andto extract the key needed for deciphering the encrypted message.

In an alternative embodiment, a message indexing mechanism is used totransmit the C&C message. In this embodiment, all entities incommunication have a list or menu of messages stored prior tocommunicating, or alternatively have access to the list of storedmessages. Each message is indexed, therefore only indices of messagesare communicated, thus a transmitting entity needs only to transmit anindex of the message, and the receiving entity has a look up table toretrieve a message from the stored list with the index, corresponding tothe index of the transmitted message. The index of the message, in thisembodiment, is inserted within the encrypted message field 830. Themessage indexing mechanism may be used as a means to limit the size ofthe ADS-B UAT frame, because the size of the index is smaller than thesize of the message itself. Exemplary values of indices may be numericalsuch as 1 to N, with each index corresponding to a message within theset of C&C messages that can be communicated between the transmittingand receiving entities. Other alphanumerical values or codes can be usedas index as well.

The index of the message may be encrypted before being inserted in theencrypted message field 830. Alternatively, the message index may not beencrypted prior to inserting within the encrypted message field 830before the transmission.

The GS control and processing system 132 can be a standalone processingsystem or integrated as part of the GS processing computer 106. GScontrol and processing system 132 can as well be implemented inhardware, in software or in combination thereof. Those skilled in theart can readily design a GS control and processing system 132 that canfulfill its functionalities described in the current disclosure. The GScontrol and processing system 132 may or may not comprise the processor625. The GS control and processing system 132 may be only Softwarestored in the memory 626 for execution by another processor such as aprocessor within the GS processing computer 106.

FIG. 9 is a signal flow diagram showing a ground station 102 and anaircraft 100 when communicating according to an embodiment of thepresent invention. As described above when a PIC enters a commandthrough the GUI 200, the C&C message is received by the GS control andprocessing 132 at step 910 and at step 920 the encryption module iscalled upon to perform an encryption according to the method of thisinvention that will be described with regards to FIG. 10A below. At step930 a header 820 is generated as described above with respect to theHeader Generator 624 of FIG. 6.

The ADS-B formatter/parser 621, at step 940 forms an ADS-B UAT frame asdescribed previously. In one embodiment the ADS-B UAT frame is formattedaccording to the frame structure shown in FIG. 8A. Alternatively theframe is formatted according to the frame structure of FIG. 8B.

Once the frame is formatted, it is transmitted through the GS ADS-Btransceiver 120 over the link 121 to the UA 100. At step 950 the frameis received and decoded at the UA 100. At step 960 the header 820 of thedecoded frame is read by the Onboard Control and processing system 124in the UA 100. As stated above the use of the binary “110” or “111” RAQ(element 730 a or element 730 b, respectively) indicates an operationaccording to the principles of the invention. The Onboard Control andprocessing system 124 upon determining that the frame complies with thestructure of this invention from the reading of the header 820, canstart the parsing of the payload portion 800 of the ADS-B UAT frameusing the ADS-B frame formatter/parser 621 (step 970). Based on thepayload type code, the Onboard Control and processing system 132 canidentify all the subfields within the payload 800 and ascertain throughthe Destination Address field 840 d that the message is addressed to theUA 100. The Onboard Control and processing system 132 can then extractthe public key of the sender to start the decryption of the encryptedC&C message within the encrypted message field 830 at step 980. Thedecryption operation will be described with regards to FIG. 10B. At step990 the decrypted message is forwarded to the Autopilot 104 of the UA100 to respond to the command conveyed by the C&C message as describedabove.

FIGS. 10A and 10B describe the encryption/decryption operationsaccording to an embodiment of the present invention. The encryption isbased on a 2-layer encryption and will be described with reference toFIG. 10A. The C&C message received at step 1010 is encrypted with the GSprivate key at step 1020 for a first layer encryption referred to asCYPHER 1 and at step 1030 a second layer encryption is applied using theUA public key to form an encrypted message or CYPHER 2. The encryptedmessage or CYPHER is inserted in the encrypted message field 830 as partof forming the ADS-B UAT frame.

The decryption operation performed by the UA 100 is shown in FIG. 10B.The received encrypted message (CYPHER 2) within the ADS-B UAT frame atstep 1040 is first decrypted with the UA private key to determine CYPHER1 at step 1050. At step 1060 the GS public key retrieved from the Publickey field 840 c of the ADS-B UAT frame is used to perform the 2^(nd)layer decryption of CYPHER 1 to extract the C&C message.

According to an embodiment of the invention, both aircraft and groundcontrol stations are equipped with 2 sets of encryption keys: public(short) and private (long). Public and private keys are computedbeforehand, and could be assigned upon aircraft registration, prior tothe specific mission or any other way known to those skilled in the art.Every user transmits its short key within the reserved bits of the ADS-Bmessage as described above, therefore, those keys can be received andstored in the memory by any other entity.

The suggested asymmetric encryption method has a “space” advantage overother symmetric methods: it can be used to encrypt a message without theneed to exchange a secret key separately. For example, Ground Stationcan send an encrypted message to the UA without any prior exchange ofsecret keys. GS just uses Aircraft's public key to encrypt the messageand Aircraft decrypts it, using its private key. Considering theincrease in air traffic, this approach is advantageous because lessstorage/memory is needed to store all the keys. When new user is added,it only needs a private and a public key, thus for n users, only 2n keysare needed. Complexity is O(n). As compared to symmetric methods, everytime a new user is added to the system, it needs to share a new key witheach previous user. For n users we have n(n−1)/2 keys needed. This iscomplexity O(n*n).

Specific issue with encrypting ADS-B messages is that ADS-B UAT messagesare only 34 bytes long (FIG. 9). Asymmetric encryption methods, assuggested above, have a lot of advantages, but compared to symmetricmethods do not guarantee not going over 34 bytes. Encrypting the wholeADS-B frame would require modifying existing technical standards toaccommodate a new size of the ADS-B frame. The solution provided abovethrough partial encrypting of the ADS-B frame allows to mitigate suchissue and limit the ADS-B UAT frame as formed to 34 bytes. Thisencryption method is an efficient way of encryption of the specificfield of ADS-B message considering small size of the ADS-B frame, whilemaintaining high security and being advantageous in large amount of airtraffic.

In one further embodiment, the Onboard control and processing system 124records various flight data for the unmanned aircraft 100, which isstored in a non-volatile memory for post flight analysis, or forpost-recovery of the UA 100 in the case of a downed UA 100 from eitherintended or unintended flight termination. In a further embodiment, theUA transceiver 122 is equipped with back-up batteries to enable acontinuous intermittent transmission of the location of a downed UA 100for recovery.

Methods and systems of the present invention can be applied in variousenvironment including any navigating entity either airborne, seaborne oron the ground such as aerial, marine or ground transportation vehicles.

The communication can be as well bidirectional between ground station toa group of entities or to an individual entity or from a seaborne orairborne entity to a ground station or land vehicle. Differentcombinations and arrangement are contemplated under the currentdisclosure and are known to those skilled in the art.

It should be noted that methods and systems of the embodiments of theinvention and data sets described above are not, in any sense, abstractor intangible. Instead, the data is necessarily presented in a digitalform and stored in a physical data-storage computer-readable medium,such as an electronic memory, mass-storage device, or other physical,tangible, data-storage device and medium. It should also be noted thatthe currently described data-processing, data-control and data-storagemethods cannot be carried out manually by a human analyst, because ofthe complexity and vast numbers of intermediate results generated forprocessing and analysis of even quite modest amounts of data. Instead,the methods described herein are necessarily carried out by electroniccomputing systems having processors on electronically or magneticallystored data, with the results of the data processing and data analysisdigitally stored in one or more tangible, physical, data-storage devicesand media.

Methods and systems of the present invention have tangible and practicaladvantages, providing more expedient and more reliable flight control ofunmanned aircrafts.

Although specific embodiments of the invention have been described indetail, it should be understood that the described embodiments areintended to be illustrative and not restrictive. Various changes andmodifications of the embodiments shown in the drawings and described inthe specification may be made within the scope of the following claimswithout departing from the scope of the invention in its broader aspect.

What is claimed is:
 1. A transmission method from a first entity to asecond entity via an Automatic Dependent Surveillance-Broadcast (ADS-B)transmission protocol, comprising: generating a message for transmissionfrom the first entity to the second entity; encrypting said message witha unique set of keys associated with the first entity and the secondentity to generate an encrypted message; generating a header foridentifying a message type; generating a sub-header having at least oneof a bitmap, a target indicator, a public key and a destination address;and forming an ADS-B Universal Access Transceiver (UAT) frame comprisinga header portion and a payload portion wherein said header is insertedinto the header portion and said encrypted message and said sub-headerare inserted into the payload portion; and transmitting said ADS-B UATframe to said second entity, thereby converting the ADS-Broadcasttransmission protocol into a unicast/multicast transmission.
 2. Themethod of claim 1 wherein the encrypted message and sub-header arecarried on reserved bits of an ADS-B UAT frame having a payload typevalue comprised between 7 and
 10. 3. The method of claim 1 wherein theencrypted message and sub-header are carried on a reserved message of anADS-B UAT frame having a payload type value comprised between 11 and 29.4. The method of claim 1 wherein said first entity is one of a groundstation and an Authorities control station and said second entity is oneof an aircraft.
 5. The method of claim 1 wherein said first entity isone of a ground station and an Authorities control station, and saidsecond entity is a group of aircraft.
 6. The method of claim 1 whereinsaid first entity is an aircraft and said second entity is one of aground station and an Authorities control station.
 7. The method ofclaim 1 wherein said encrypting step comprises first encrypting themessage with the first entity private key to generate a first encryptedmessage and then encrypting the first encrypted message with the secondentity public key to generate the encrypted message and wherein saidunique set of keys comprise the first entity private key and the secondentity public key.
 8. The method of claim 1 wherein said encrypting stepcomprises first encrypting the message with the second entity public keyto generate a first encrypted message and then encrypting the firstencrypted message with the first entity private key to generate theencrypted message and wherein said unique set of keys comprise the firstentity private key and the second entity public key.
 9. The method ofclaim 1 wherein said sub-header comprises the bitmap, the targetindicator and the public key of the first entity.
 10. The method ofclaim 1 wherein the message is one of a flight control command, anemergency command and an informational message.
 11. The method of claim1 wherein the message is selected from a menu of predetermined messages.12. A system for communication between a first entity and a secondentity via Automatic Dependent Surveillance-Broadcast (ADS-B)transmission protocol, comprising: an encryption module for applying a2-layer encryption to a command and control (C&C) message to generate anencrypted C&C message wherein said 2-layer encryption is based on aunique set of keys associated to the first entity and to the secondentity; a header generation module for generating a header comprising anaddress qualifier; an ADS-B Universal Access transceiver (UAT) frameformatter for generating a sub header comprising at least one of abitmap, a target indicator, a public key and a destination address; saidADS-B UAT frame formatter further forming an ADS-B UAT frame comprisinga payload portion and a header portion wherein said header portionincludes said generated header and said payload portion includes saidsub-header and said encrypted C&C message and wherein said addressqualifier indicates that said ADS-B UAT frame carries a C&C message; anda transceiver for transmitting said ADS-B UAT frame to the secondentity.
 13. The system of claim 12 wherein said first entity is one of aground station and an Authorities control station and said second entityis one of an aircraft.
 14. The system of claim 12 wherein said uniqueset of keys comprise a private key of the first entity and a public keyof the second entity.
 15. The system of claim 14 wherein the private keyis applied on the C&C message to generate a first encrypted message andthe public key is applied on said encrypted message to generate theencrypted C&C message.
 16. The system of claim 12 wherein the encryptedC&C message and sub-header are carried on reserved bits of an ADS-B UATframe having a payload type value comprised between 7 and
 10. 17. Thesystem of claim 12 wherein the encrypted C&C message and sub-header arecarried on a reserved message of an ADS-B UAT frame having a payloadtype value comprised between 11 and
 29. 18. The system of claim 12wherein the C&C message is one of a flight control command, an emergencycommand and an informational message.
 19. The system of claim 12 furthercomprising a control processing system at the second entity forreceiving said transmitted ADS-B UAT frame.
 20. The control processingsystem of claim 19 comprising an ADS-B frame parser for parsing theADS-B UAT frame received at said second entity.
 21. The controlprocessing system of claim 19 comprising a decryption module fordecrypting the encrypted C&C message to retrieve the C&C message andsaid control processing system further forwards said C&C message to anautopilot of the second entity.
 22. A transmission method from a groundstation to an aircraft via an Automatic Dependent Surveillance-Broadcast(ADS-B) transmission protocol, comprising: generating a command andcontrol (C&C) message for transmission to the aircraft responsive to adetection of a radio link loss situation between the ground station andthe aircraft; encrypting said C&C message with a unique set of keysassociated to the ground station and to the aircraft to generate anencrypted C&C message; generating a header for identifying a messagetype, wherein an address qualifier of said header indicates the messagetype as a C&C message; generating a sub-header having at least a bitmap,a target indicator and a public key; and forming an ADS-B UniversalAccess Transceiver (UAT) frame comprising a header portion and a payloadportion wherein said header is mapped into the header portion and saidencrypted C&C message and said sub-header are mapped into the payloadportion; and transmitting said ADS-B UAT frame to said aircraft therebyconverting the ADS-Broadcast transmission protocol into aunicast/multicast transmission.
 23. The method of claim 22, furthercomprising: provided a size of said ADS-B frame carrying the encryptedC&C message exceeds a size of a standard ADS-B UAT frame, limiting thesize of said ADS-B UAT frame carrying the encrypted C&C message to fitinto the size of the standard ADS-B UAT frame.
 24. The method of claim22, wherein the generating further comprises selecting the C&C from alist of predetermined C&C messages.